1. Field of the Invention
The present invention is within the field of voltage-to-frequency converters (V/F converters). In particular, the invention comprises a closed-loop V/F converter including a voltage-controlled oscillator ("VCO") configured in a feedback voltage-locked loop. This V/F converter is especially useful in combination with a phase-locked loop ("PLL") frequency synthesizer for an RF receiver of a wireless radio.
2. Description of the Related Art
A primary constraint on the design of a phase-locked loop frequency synthesizer in a wireless radio is its phase-noise performance. The phase-noise of the PLL, generally, is dominated by the phase-noise of the constituent VCO in the range of interest. The phase-noise of the VCO is inversely proportional to its power consumption. Thus, in order to meet the phase-noise requirements for the PLL, it is generally necessary to use expensive, high-quality VCO circuits, and/or it is necessary to increase the power of the VCO, which is disadvantageous in portable radio devices where power is limited.
A free-running VCO is essentially an open-loop system, where any noise in the system directly affects the output with no means of correction. When configured in a phase-locked loop, which is a closed-loop feedback system, the loop corrects the noise of the VCO within its bandwidth. Thus the VCO phase-noise is inhibited within the PLL bandwidth.
In this type of system, it is beneficial to increase the PLL bandwidth as much as possible. But, the maximum PLL bandwidth is limited by the spurious rejection of the loop. Hence, the PLL bandwidth is generally one order of magnitude less than the reference frequency. In a classical PLL-based frequency synthesizer in a wireless transceiver, the reference frequency would be equal to the channel spacing, and the loop does not inhibit the VCO phase-noise in the range of interest.
Therefore, a new architecture for a frequency synthesizer incorporating a VCO is needed.